Cd1−xZnxTe thin films were prepared on 7059 Corning glass substrates using an rf magnetron sputtering system and CdTe(1−y)+ZnTe(y) targets. The concentration (x) of Zn in the films did not coincide with the relative weight (y) of the ZnTe powder in the compressed targets. Values of x were in the range 0–0.30 as determined from x-ray diffraction patterns. The band gap energy (Eg) of the Cd1−xZnxTe samples was calculated from the photoreflectance spectra measured on the films. The position of the experimental points in the Eg versus x plot show a deviation from the phenomenological quadratic relation Eg=Eg0+ax+bx2 calculated within the virtual crystal approximation (VCA). The depart of the Eg values for higher Zn concentrations from the expected VCA model is probably due to a larger clustering of Zn atoms and/or a percolation phenomena. We obtain fitted values for the parameters a and b within the VCA approach. Comparison with data reported by other authors is made.

On the bowing parameter in Cd1−xZnxTe

We present a current status of the development of hard x-ray telescope using Pt/C multilayer supermirror. The telescope system is to be made by combining thin foil replication technology for high throughput mirror and multilayer supermirror coating technology for hard x-ray reflection. After the successful multilayer coating on the replica foil mirror, we made the performance demonstration model of this type of telescope, having 20 replica foil supermirrors, 10 primary and 10 secondary reflectors, with focal length of 4.75 m and radius of 100 mm. Pt/C multilayer supermirror structure was designed and optimized to have high and flat reflectivity for x-ray energy from 25 through 40 keV. After some efforts to avoid heat damage of replica foil mirror during the deposition process of multilayer by DC sputtering system, we could establish the fabrication method of supermirror structure on replica foil mirror. Based on the x-ray measurement, we found that this demonstration model showed the half power diameter of 1.9 arcmin for had x-rays and nearly the same reflectivity and energy band width as expected. In this paper, we present the design of graded multilayer as the supermirror, the fabrication and the performance of this demonstration model.

Development of a multilayer supermirror for hard x-ray telescopes

We report results of an experiment conducted in May 1997 to measure CdZnTe background and background reduction schemes in space flight conditions similar to those of proposed hard x-ray astrophysics missions. A 1 cm2 CdZnTe detector was placed adjacent to a thick BGO anticoincidence shield and flown piggy backed onto the EXITE2 scientific balloon payload. The planar shield was designed to veto background counts produced by local gamma-ray production in passive material and neutron interactions in the detector. The CdZnTe and BGO were partially surrounded by a Pb-Sn-Cu shield to approximate the grammage of an x-ray collimator, although the field of view was still approximately 2 (pi) sr. At an altitude of 127000 feet we find a reduction in background by a factor of 6 at 100 keV. The non-vetoed background is 9 by 104 cts cm-2s-1 at 100 keV, about a factor of 2 higher than that of the collimated EXITE2 phoswich detector. We compare our recorded spectrum with that expected from simulations using GEANT and find agreement within a factor of 2 between 30 and 300 keV. We also compare our results with those of previous experiments using passive lead and active NaI shields, and discus possible active shielding schemes in future astronomy mission employing large arrays of CdZnTe detectors.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

/pdf/cdznte-background-measurement-at-balloon-altitudes-with-an-22mbiq2oyu.pdf

CdZnTe Background Measurement at Balloon Altitudes with an Active BGO Shield

We have investigated the ionic relaxations, electronic structures, and optical properties for ${\mathrm{Cd}}_{1\ensuremath{-}x}{\mathrm{Zn}}_{x}\mathrm{Te}$ alloys using density functional theory. The quasi-zinc-blende structure is used with special emphasis on the relaxation behaviors of ${\mathrm{Te}}^{2\ensuremath{-}}$ around either ${\mathrm{Cd}}^{2+}$ or ${\mathrm{Zn}}^{2+}$. Our calculations confirm that the relaxations of the anion rather than the cation contribute primarily to the alloying process as predicated by the experiments. The differences in the ionicity of ${\mathrm{Cd}}^{2+}$ and ${\mathrm{Zn}}^{2+}$ and their configurations around ${\mathrm{Te}}^{2\ensuremath{-}}$ are responsible for the different relaxation behaviors of ${\mathrm{Te}}^{2\ensuremath{-}}$. A striking result is the relevance of the relaxation behaviors of ${\mathrm{Te}}^{2\ensuremath{-}}$ with the alloying effect on the electronic states. This result supports the electronic features of ${\mathrm{Cd}}_{0.5}{\mathrm{Zn}}_{0.5}\mathrm{Te}$ alloy reported by the systematic analyses with quasirandom structure. The band structures obtained here are used to determine the optical functions. The comparison with the available experimental and theoretical results suggests an overall topological resemblance in the present dielectric function spectra when the band-gap correction is included.

Composition-dependent electronic properties, optical transitions, and anionic relaxations of Cd 1-x Zn x Te alloys from first principles

A CdZnTe strip detector array with capabilities for arc second imaging and spectroscopy is being developed for a space flight gamma-ray burst instrument. Two dimensional strip detectors with 100 micrometers pitch have been fabricated and wire bonded to readout electronics to demonstrate the ability to localize 22 to 122 keV photons to less than 100 micrometers. In addition, good spectral resolution has been achieved. The uniformity of response and relative efficiency of the strip detector will be discussed. Results form electrical characterization which include strip leakage current and strip capacitance will be presented.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

CdZnTe strip detector for arcsecond imaging and spectroscopy

Because of its high atomic number and convenient room temperature operation, CdZnTe has great potential for use in both balloon and space borne hard x-ray (5 - 200 keV) astrophysics experiments. Here we present preliminary results from the first CdZnTe background measurements made by a balloon instrument. Measurements of the CdZnTe internal background are essential to determine which physical processes make the most important background contributions and are critical in the design of future scientific instruments. The PoRTIA CdZnTe balloon instrument was flown three times in three different shielding configurations. PoRTIA was passively shielded during its first flight from Palestine, Texas and actively shielded as a piggyback instrument on the GRIS balloon experiment during flights 2 and 3 from Alice Springs, Australia. PoRTIA flew twice during the Fall 1995 Alice Springs, Australia campaign using the thick GRIS NaI anticoincidence shield. A significant CdZnTe background reduction was achieved during the third flight with PoRTIA placed completely inside the GRIS shield and blocking crystal, and thus completely surrounded by 15 cm of NaI. These background results are presented and contributions from different background processes are discussed.

CdZnTe background measurements at balloon altitudes

A CdZnTe detector array is being developed for the proposed gamma-ray Burst ArcSecond Imaging and Spectroscopy (BASIS) spaceflight mission to accurately locate gamma-ray bursts, determine their distance scale, and measure the physical characteristics of the emission region. Two-dimensional strip detectors with 100 μm pitch have been fabricated and wire bonded to readout electronics to demonstrate the ability to localize 60 and 122 keV gamma-rays to less than 100 μm. Radiation damage studies on a CdZnTe detector exposed to MeV neutrons showed a small amount of activation but no detector performance degradation for fluences up to 10 10 neutrons/cm 2 . A 1 × 1 in. CdZnTe detector has also been flown on a balloon payload at 115 000 ft in order to measure the CdZnTe background rates.

CdZnTe detectors for gamma-ray Burst ArcSecond Imaging and Spectroscopy (BASIS)

Gamma-ray observations in the medium energy range (0.50-50.0 MeV) are central to unfolding many outstanding questions in astrophysics. The challenges of medium-energy gamma-ray observations, however, are the low photon statistics and large backgrounds. We review these questions, address the telescope technology requirements, and describe our development of the 3-Dimensional Track Imaging (3-DTI) Compton telescope and its performance for a new medium-energy gamma-ray mission. The 3-DTI is a large-volume time projection chamber (TPC) with a 2-dimensional gas micro-well detector (MWD) readout.

/pdf/medium-energy-gamma-ray-astrophysics-with-the-3-dti-gamma-1pt5z9vxvz.pdf

Medium-Energy Gamma-Ray Astrophysics with the 3-DTI Gamma-Ray Telescope

In this paper we report on the design and performance of a new Application Specific Integrated Circuit (ASIC) for use with silicon strip detectors. This ASIC chip was designed for the positron–Electron Magnet Spectrometer experiment (POEMS), which has been proposed as a NASA small explorer payload. The ASIC performs as designed, with a noise figure of 800 electrons ENC with a 10 pF load, while using only 170 μ W per channel.

The POEMS strip detector ASIC chip

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CdZnTe background measurements at balloon altitudes

CdZnTe detectors for gamma-ray Burst ArcSecond Imaging and Spectroscopy (BASIS)

Medium-Energy Gamma-Ray Astrophysics with the 3-DTI Gamma-Ray Telescope

The POEMS strip detector ASIC chip